A band gap reference voltage source is disclosed by the semiconductor circuitry text book "Halbleiter-Schaltungstechnik" by U. Tietze and Ch. Schenk published by Springer Verlag, 9th edition, pages 558 et seq. In this known band gap reference voltage source the base-emitter voltage of a bipolar transistor is employed as the voltage reference. The temperature coefficient of this voltage of -2 mV/K is markedly high for the voltage value of 0.6 V. Compensating this temperature coefficient is achieved by adding to it a temperature coefficient of +2 mV/K produced by a second transistor. It can be shown that by operating the two transistors at differing current densities a highly accurate reference voltage of 1.205 V can be achieved which exhibits no dependency on temperature.
This known band gap reference voltage source has the disadvantage, however, that its temperature independence applies only for a certain supply voltage. This is due to the so-called Early effect which manifests itself by the collector current being a function of the collector emitter voltage of a transistor. When there is a change in the supply voltage of the known band gap reference voltage source, therefore, the current values in the individual branches of the circuit change so that the current ratios necessary for achieving temperature compensation no longer apply. The generated reference voltage is accordingly no longer independent of the temperature.
One way of solving this problem would be to generate the currents needed by means of current mirrors, for which proposals already exist, to more or less completely eliminate the influence of the Early effect. Such compensated current mirror circuits are disclosed for instance in the textbook on integrated bipolar circuits "Integrierte Bipolarschaltungen" by H.-M. Rein, R. Ranfft, published by Springer Verlag 1980, pages 250 et seq. for bipolar transistors. For current mirrors comprising field-effect transistors, circuits for eliminating the Early effect--also termed lambda effect in conjunction with literature on field-effect transistors--are described in "CMOS Analog Circuit Design" by Phillip E. Allen and Douglas R. Holberg, Holt, Rinehart and Winston, Inc. pages 237 et seq.
One drawback of using compensated current mirrors to generate the currents required in a band gap reference voltage source is that it is no longer possible to operate such compensated current mirrors with voltages of less than 3 V. This results from the physical parameters of the semiconductor elements used which require certain minimum voltages (voltage V.sub.BE for bipolar transistors and the threshold voltage V.sub.T for field-effect transistors) for their operation.
More recently, however, a growing need for band gap reference voltage sources capable of being operated with operating voltages of around 3 V and less has arisen, this being due to the 5 V supply voltage formerly always used in digital circuitry now being replaced more and more by a supply voltage of 3 V.
The object of the invention is based on creating a band gap reference voltage source capable of generating a precisely temperature-compensated stable reference voltage in a broad supply voltage range down to 3 V.